alnico units by JBL

tinitus

Ex-Moderator R.I.P.
2005-11-24 1:47 am
very often Alnico's are covered with a cup
mostly, if people could see the magnet, they wouldn't buy it
I guess its the reason to cover it with a cup :D

but if the magnet is visible it will be longer, with smaller diameter
ferrite magnets are flat and wide diameter
 
Most of the Alnico woofers have a cast iron pot structure on the rear. the Alnico ring is a cylinder inside the voice coil and the magnet circuit is finished by the pot. It (the pot)was smaller in diameter and smooth, with a blue grey or black crackle paint finish.

Ferrite magnets have a visible ferrite ring (looks like ceramic or dull gray glass) as the widest diameter element. Most units have it hidden under a rubber tire as it has sharp edges and looks less elegant. Those units also have a die cast back plate rather than a full cup.

I assume you are looking for ferrite versions due to their lower distortion and freedom from high power demagnetization!

David S.
 
Well the curve on alnico is rather different than ferrite, and this has some advantages - or so it is said.

Otoh, early speakers, the JBLs included were made and designed before the advent of T/S design parameters.

Also, most of the early JBL alnico drivers are relatively low power handling as well.

Iirc, many of the JBL magnet structures did have copper shorting rings. I think this is correct, my swiss cheese brain is showing some indication of a vague recollection.

_-_-bear
 
Manufactures switched to more cost effective solutions over the years:

- Heavier impregnated and/or plastic cones vs. lightweight paper cones (for low fs)
- Straight edge cones vs. hyperbolic cones (easier to manufacture)
- Pressed steel basket vs. diecast aluminium basket (cheap)
- Large diameter coil vs. small and lightweight coil (efficiency compensation)
- Black or aluminium painted cone vs. natural color paper (for aesthetics)
- Foam or rubber edge vs. impregnated fabric edge (for higher excursion, or fashion)
- Aluminium former vs. paper (for better power handling)
- Ferrite vs. AlNiCo (cost, easier to manufacture)

It seems like they are influenced by the car audio needs :eek:
All these modern "deficiencies" are easily compensated by big Watts.
 
I was at JBL shortly after the big conversion in the mid 70's. Price instability in Africa (Zaire?) made cobalt unavailable and Alnico went through the roof.

It was easy to design equivalent ferrite motors but they found that just achieving equal flux was inadequate. Midrange distortion was higher until they came out with the SFG (aluminum rings and a symmetrical gap structure). This made the distortion every bit as good as the Alnico magnets had been.

McIntosh came up with a similar structure called LD/HP and Carl VanGelder got a patent along with JBL (why the patent office issued two patents for the same thing is beyond me).

Alnico was fine with 20 watt amplifiers but anything in the hundreds can easily and instantly drop the sensitivity by 2 or 3 dB with a good LF blast.

Lots of white papers at the Harman site on this.

David S.
 

tinitus

Ex-Moderator R.I.P.
2005-11-24 1:47 am
Thanks..... on diyaudio I always find the answers...

not even close yet ;)

Alnico was fine with 20 watt amplifiers but anything in the hundreds can easily and instantly drop the sensitivity by 2 or 3 dB with a good LF blast.

better stick to 2A3 amps then ;)

btw, I would have said earlier, when messing with alnico drivers you need to know what your are doing
I just limited it to say, its for enthusiasts
but someone blowing it with 200watts obvious doen't know what he is dealing with at all

like I would have said, you need to know exactly why, where, when, and how
 
Alnico was fine with 20 watt amplifiers but anything in the hundreds can easily and instantly drop the sensitivity by 2 or 3 dB with a good LF blast.

Lots of white papers at the Harman site on this.

David S.
David,

My recollection of Alnico drivers being used with some fairly large peak power amps like Phase Linear Blue light 700s (capable of 350 watts at 8 ohm) did include power compression on the order of 3 dB, but that was not an instant thing, more like over an hour or more as the voice coil heated up the magnet structure. Larger ferrite magnet structures took longer to heat up and had more surface area to dissipate the heat from, so had less thermal compression.

I never experienced a Fender Twin loaded with D 120s that lacked any sensitivity either, and those speakers often were subjected to FTB 50 watts per cone all night, all year.

As an owner of dozens of Alnico JBL 2482s subjected to peaks of 90 watts per driver in the 1200-5000 Hz range for many years, never noticed them being less sensitive than the ceramic 2445 using the same horn.

Although vague references to Alnico demagnitization abound, a Google search of the web and the Harman site looking for white papers (there are mainly rabid guitar players blogging about Alnico) turned up none, do you have any reference material you can cite regarding Alnico demagnitization?

Art
 

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Not reference material but I have seen it done first hand. At JBL we had a Crown D600 to drive any speaker being tested on the outdoor asphalt pad. Greg Timbers showed me once how he could pull the input on it (or maybe it was via the sine generator) and blast the unit with hum for a second and then run the curve again to see what the sensitivity had become (midband level). He was trying to drop the sensitivity/raise the Q of a particular driver and, after a couple of trys got what he wanted.

Not a thermal thing but more to do with its resistance to demagnetization related to its operating point. It was a bit random because you don't know where the signal would end up with your short blast, positive or negative.

I distinctly remember both Greg's approach, and seeing the curve difference. I thought this was a well known phenomenon and that people offered remagnetizing services?

On the other side of the fence there is always ferrite losing sensitivity due to extreme cold. I've heard of speaker shipments that sat out all night in the far flung regions of Minnesota. They were returned by customers with complaints of low sensitivity.
 
Not reference material but I have seen it done first hand. At JBL we had a Crown D600 to drive any speaker being tested on the outdoor asphalt pad. Greg Timbers showed me once how he could pull the input on it (or maybe it was via the sine generator) and blast the unit with hum for a second and then run the curve again to see what the sensitivity had become (midband level). He was trying to drop the sensitivity/raise the Q of a particular driver and, after a couple of trys got what he wanted.

Not a thermal thing but more to do with its resistance to demagnetization related to its operating point. It was a bit random because you don't know where the signal would end up with your short blast, positive or negative.

I distinctly remember both Greg's approach, and seeing the curve difference. I thought this was a well known phenomenon and that people offered remagnetizing services?

On the other side of the fence there is always ferrite losing sensitivity due to extreme cold. I've heard of speaker shipments that sat out all night in the far flung regions of Minnesota. They were returned by customers with complaints of low sensitivity.
I am aware of the well known phenomenon of remagnetizing services for old Alnico, as well as speaker cables that cost over $10,000.

My experience with Alnico was mostly in Minnesota, though Showco and Clair Brothers also brought frozen truckloads of Alnico through Canada and our tundra on hundreds of national tours.
Not once in from 1974 through the 1990s when most Alnico drivers were phased out had I heard of heat, cold or power demagnitizing Alnico, though heard of heat problems (that were solved fairly early on) with Neodymium.

Why would Greg Timbers not just start with less flux density rather than trying to beat it down with a random process?
What was the impact of the short blast ending on a positive or negative cycle ?